The success of a universal influenza virus vaccine depends on two critical factors: one, the identification of novel antigens that define broadly cross-reactive epitopes that are shared by a wide range of influenza virus strains; and two, the ability of these novel vaccine constructs to induce long-term antibody responses and protective immunity. Project #1 (PI: Palese) will focus on the design of these new immunogens such as the headless HA and the long alpha helix of the HA2 molecule that are based on conserved influenza virus epitopes whereas our Project #2 will work in collaboration with Project #3 (PI: Garcia-Sastre) and the Monoclonal Antibody Technology Core (PI: Wilson) to understand how to generate long-lived antibody responses using these novel vaccine constructs. Our project will focus on humoral immunity and, in particular, on CD4 T follicular helper (TFH) cells that are critical for generating potent and long-lasting antibody responses. The following two specific aims are proposed; Specific Aim 1; To characterise human influenza virus specific CD4 T follicular helper cell responses and to identify conserved CD4 T cell epitopes that will be incorporated into the universal influenza vaccine. In this aim we will identify conserved epitopes that stimulate strong CD4 TFH responses in a broad cross-section of the human population and ensure that these sequences are present within the vaccine candidates, so that sufficient T cell help can be provided to stimulate robust and long-lived antibody responses. The hypothesis to be tested in Aim 1 is that long-term humoral immunity is critically dependent on CD4 TFH cells and that the efficient generation of these cells is an essential and obligatory component of an effective influenza virus vaccine. Specific Aim 2: To optimize immunogenicity of the universal influenza virus vaccine. IL-21, a key cytokine produced by CD4 TFH cells, is essential for the generation of long-lived anti-viral antibody responses. The hypothesis that we will test in this aim is that adjuvants (cytokine adjuvants, nanoparticles containing TLR4/TLR7 or a combination) that specifically enhance CD4 TFH responses or IL-21 signaling or are a critical component of a universal influenza vaccine inducing long-term protective immunity. RELEVANCE (See instructions): The overall goal of these studies is to develop a universal influenza vaccine that would provide long-term protective immunity against a broad range of influenza virus strains. Such a novel vaccine would greatly reduce the mortality and morbidity seen with this infection.